tests: switch to pytest as the default test runner. See #153

Merged Christoph Reiter requested to merge creiter/pygobject:tests-use-pytest into master

The TEST_NAMES env var gets translated to work with the pytest syntax (foo.py::class::method).

Rename one class which triggers a pytest warning because it starts with "Test" but isn't one.

Remove erroring out on Python warnings as pytest triggers some deprecation warnings by default.

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